Polymeric silicone coatings and, to a lesser extent, silicone-organic coatings, have long been appreciated in the art for their ability to withstand high temperatures and the ravages of harsh climatic environments. The silicone resins have found particular utility in electrical insulation applications, as additives for organic paints and coatings and, when combined with minor portions of a silicone fluid, as excellent release coatings in bakeware applications. Silicone resins have also been successfully added to, e.g., alkyd compositions to improve weather resistance of the latter.
The various silicone resins, even to this day, are generally marketed in solution form despite ever increasing antipollution sentiment throughout the world. Just as in the case of various organic coating formulations, efforts to produce environmentally acceptable silicone resin coating compositions, having properties comparable to those delivered from conventional organic solvents, have met with only limited success to this point. Yet, some inroads toward the environmental goal of reduced volatile organic compounds (VOC) emission have been achieved with the advent of water-based systems. Much of the progress in this area can be attributed to the discovery of various emulsifying agent combinations which provide good, stable emulsions of the silicone resins and fluids in water.
Thus, for example, U.S. Pat. No. 4,501,619 to Gee discloses aqueous emulsions of carboxyl-containing silicone fluids which rely on a surfactant combination consisting of (1) a primary surfactant containing a polyalkylene oxide chain having a lipophilic terminal group; and (2) a cosurfactant selected from fatty acid esters of sorbitol or a polyalkylene chain having a lipophilic terminal group.
Traver, in U.S. Pat. No. 4,518,727, teaches the preparation of water-based silicone resin emulsions which employ a surfactant combination consisting of a cellulosic emulsification agent and an anionic surfactant.
A novel oil-in-water emulsion system, particularly suitable for dispersing silicone oils having a high viscosity, is disclosed by Narula in U.S. Pat. No. 4,788,001, assigned to the assignee of the present invention. In this case, a three-component surfactant combination was shown to be advantageous when each surfactant had a critical, but distinct, hydrophilic/hydrophobic balance (HLB) value.
Emulsions prepared according to the above cited patent to Narula were combined with various aqueous organic polymers to form water-based silicone-organic compositions in U.S. Pat. No. 4,803,233 to Narula and Swihart, assigned to the assignee of the present invention.
Aqueous silicone-containing coating compositions for high temperature appliances were disclosed in U.S. Pat. No. 4,855,348 to Strader. Here, a stable paint was produced from a solid silicone resin and an ammonium salt of a drying oil-modified acrylic copolymer. The paint can be applied to a metal substrate and fired to burn off the acrylic resin and thereby provide a continuous coating capable of withstanding temperatures up to 1,300.degree. F.
Blizzard, in U.S. Pat. No. 4,423,095, discloses silicone-organic coating compositions consisting essentially of a homogeneous mixture of certain silicone resins, one or more silicon-free, film-forming organic polymers and a compatibilizing liquid. The organic polymer can be a water-soluble material, such as poly(vinyl alcohol) or poly(acrylic acid), or water-dispersed material, such as poly(methyl methacrylate) or poly(vinyl acetate). The compatibilizing liquid can be water or an organic liquid, as desired. The silicone resin is said to be the reaction product of a liquid organohydrogenpolysiloxane and a resinous copolymeric siloxane containing silicon-bonded hydroxyl radicals and consisting essentially of R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units.
There are distinct drawbacks to the above described silicone-organic aqueous systems. Although aqueous silicone emulsions taught by Narula represent a significant advancement in the art, the quantity and/or type of emulsifying agents required can severely impair the high temperature stability of coatings formed from these systems, as when combined with organic polymers according to the methods outlined by Narula and Swihart, cited supra. Likewise, the compositions produced by Strader, also cited supra, require the burning off of the acrylic resin contained therein to produce heat resistant coatings.